Operations, such as surveying, drilling, wireline testing, completions, production, planning and field analysis, are typically performed to locate and gather valuable downhole fluids. Surveys are often performed using acquisition methodologies, such as seismic scanners or surveyors to generate maps of underground formations. These formations are often analyzed to determine the presence of subterranean assets, such as valuable fluids or minerals, or to determine if the formations have characteristics suitable for storing fluids. Although the subterranean assets are not limited to hydrocarbon such as oil, throughout this document, the terms “oilfield” and “oilfield operation” may be used interchangeably with the terms “field” and “field operation” to refer to a field having any types of valuable fluids or minerals and field operations relating to any of such subterranean assets.
During drilling and production operations, data is typically collected for analysis and/or monitoring of the operations. Such data may include, for instance, information regarding subterranean formations, equipment, and historical and/or other data.
Various equipment may be positioned about the field to monitor field parameters, to manipulate the operations and/or to separate and direct fluids from the wells. Surface equipment and completion equipment may also be used to inject fluids into reservoirs, either for storage or at strategic points to enhance production of the reservoir.
It is well known that mechanical disturbances can be used to establish acoustic waves in earth formations surrounding a wellbore, and the properties of these waves can be measured to obtain information about the formations through which the waves have propagated. In geophysics, a Vertical Seismic Profile (VSP) refers to measurements made in a wellbore using geophones inside the wellbore and a source at the surface near the well. Typical applications of VSP measurements include obtaining images of higher resolution than surface seismic images and looking ahead of the drill bit during drilling operations. The acquisition geometry of VSPs may vary in the well configuration, the number and location of sources and geophones, and how the sources and geophones are deployed. Most VSPs use a surface seismic source, which is commonly a vibrator on land and an air gun in offshore or marine environments. Different types of VSPs include the zero-offset VSP, offset VSP, walkaway VSP, walk-above VSP, salt-proximity VSP, shear-wave VSP, and drill-noise or seismic-while-drilling VSP.
Seismic inversion is the process of transforming seismic reflection data into a quantitative rock property description of a reservoir (e.g., acoustic impedance, shear impedance, and density). Seismic inversion typically includes other reservoir measurements such as well logs and cores that contribute low frequency information below the seismic band and to constrain the inversion. VSP data is typically underutilized in seismic inversion workflows leading to results with unknown errors. For example, VSP-seismic inversion typically uses processed VSP data in the form of a one dimensional (1D) seismic trace (i.e., a corridor stack) by assuming a normal incidence angle with respect to subterranean reflectors (e.g., formation layers) and using downgoing waveform as a wavelet with no account for anelastic effects (e.g., caused by fine layering, pore fill, rock itself esp. clays) on the wavefield. As a result, typical VSP seismic inversion generates 1D traces representing the acoustic impedance and lacking shear impedance and density information.
The normal incidence angle assumption may cause the estimate for acoustic impedance to be less accurate and, in some cases, cause the estimate to be invalid when the actual angle of incidence differs more than 15 degrees or so from the normal incidence assumption. In addition, using the downgoing wavefield as input wavelet has proven to be incorrect in that an extracted upgoing wavelet (by comparing upgoing wavefield to logs) is often rotated, which can have significant impact to the inversion result. Furthermore, attempt to apply VSP-seismic inversion workflow to deviated wells have been unsuccessful.